The Essence of OOP: It's all messages, all the time.

Category Archives: DLR

When I started work on Essence#, on or around 9 Jan 2014, I had never heard of Iron Smalltalk. That name, of course, would be a rather obvious choice, given names such as “Iron Python,” “Iron Ruby” and “Iron Scheme” for other DLR-hosted languages. And yes, I not only considered using “IronSmalltalk” as the language’s name, I actually did use it initially.

Nor do I believe that either of my two advisers/consultants on the project (Craig Latta and Peter Lount) had ever heard of an actual ‘IronSmalltalk’ either (as a real implementation of the language, and not as a concept.) If they had, they certainly didn’t mention it. And Craig Latta was in the middle of a multi-week visit here with me where I live during the time I started the Essence# project, and I was speaking by phone with Peter Lount on a daily basis about the project (and we’re still doing that.) So both of them would have had plenty of chances to tell me about the “other” IronSmalltalk while I was still using that name for what will now always be known as Essence#.

In any case, by mid February (2014,) I changed the name–not because I had discovered that there already was an IronSmalltalk, but because I didn’t want to have the word “Smalltalk” in the language’s name (but this post isn’t about that, so I’ll explain why I came to that view some other time.)

Fast forward to late May 2014: That’s when I discovered that there was an actual Smalltalk implementation based on the DLR other than mine, and that it was named IronSmalltalk.

Even now, I don’t know all that much about it. I haven’t browsed the code other than to have looked at the folders and file names using the CodePlex source code browsing applet.

Why not? Partly because I’m just too busy implementing Essence#, partly because it’s almost certainly way too late to make any significant architectural changes to Essence# based on whatever I might learn by reading Todor Todorov’s code (he’s the author,) and partly because I just don’t want to plagiarize it (even though it’s open source.)

Most of what I know about IronSmalltalk’s design and implementation I learned just by watching the video of Todor Todorov’s ESUG 2011 presentation on IronSmalltalk. From that, I can tell that the author of IronSmalltalk a) knows what he’s talking about, b) made some of the same architectural decisions I did, but c) did some things rather differently.

For example, IronSmalltalk uses CLR Strings as the direct implementation of Smalltalk Strings, but Essence# does not. The reason is because CLR Strings are intrinsically immutable. Yes, the ANSI Smalltalk Standard requires that String literals be immutable–but that’s not a problem for Essence#, because any Essence# object can be made immutable. And Strings have traditionally been mutable in Smalltalk. Interestingly, IronSmalltalk and IronPython both decided to adopt CLR Strings as their native String objects, while IronRuby and Essence# both decided to implement their own Strings. Although Essence# can and does use CLR Strings also (and I assume IronRuby does the same.)

Just so you know: The primary reason that Essence# doesn’t use CLR Strings and doesn’t use CLR arrays as its “native” implementation of Strings or of Arrays is because the #become: primitive is intrinsically not implementable on the CLR. By implementing both Strings and Arrays as a wrapper over native CLR arrays, much of the pain of not having a #become: primitive is eliminated: The double indirection makes it possible to resize Strings and Arrays “in place” without needing to use #become:. The fact that Smalltalk Strings have traditionally been mutable was a secondary consideration.

Another example involves the DLR’s dynamic binding protocol: IronSmalltalk does it the way the DLR documentation strongly recommends, and Essence# does not. So what’s the recommended way, and why doesn’t Essence# do it that way? Glad you asked:

The officially recommended (“canonical”) protocol for dynamic binding using the DLR’s DynamicMetaObject Protocol can be expressed as follows:

1. Each operand of an operation (e.g., a message send, although in other languages there are many other possibilities, because most languages do so many things using special syntax) is asked to provide a DynamicMetaObject to act as its agent for participating in the DLR’s meta-object protocol for dynamic binding. If the object is unable to do so, a default DynamicMetaObject is created for it (sort of like a court-appointed public defender.)

2. The DynamicMetaObject that is the agent for the object that is responsible for dynamically (at run time) determining the semantics of the operation is asked to bind the abstract operation (whatever that may be) to a specific physical implementation. Note that identifying which object that is–or perhaps which set of objects–is the responsibility of each DLR-based language’s dynamic binding subsystem.

For example, in the case of most OO languages, the DynamicMetaObject that is the agent for the object that is the receiver of a message would be asked to provide a binding for that message–in other words, an invocable function and associated “binding restriction.” A “binding restriction” is a predicate that can be evaluated to discover whether the binding (the invocable function) is still valid, given the current set of operands. Typically, if the “type” or “class” of the “receiver” isn’t the same as it was when the binding was computed, then the binding is no longer valid and must be recomputed.

3. If the DynamicMetaObject is able to provide a binding, then that binding will be used as the physical implementation of the abstract (logical) operation, for as long as the binding restriction predicate says that the binding is still valid.

4. However, if the DynamicMetaObject of the object nominally responsible for defining the semantics of an operation is not able to provide a binding (e.g, what’s the semantics of the DLR-standard “DeleteMember” abstract operation when the receiver is a Smalltalk object, or of the DLR-standard “Invoke” abstract operation when the receiver is null?), then the responsibility for providing a binding is redirected to the dynamic binding subsystem of the language that compiled the code that’s undergoing dynamic binding. This “host language” (to use the term the DLR uses) may be able to provide a binding, or it too may fail.

5. If the host language can provide a reasonable binding based on its own rules and semantics, it will do so. Otherwise, as a last resort, the binding that will be used would typically report or raise an error (although the DLR imposes no such requirement.) Note that different host languages may bind the exact same abstract operation on the exact same object differently: For example, one language might bind any operation applied to null by providing an invocable function that raises the NullReferenceException, whereas another language might instead provide a method it finds in the method dictionary of its UndefinedObject class, and yet another language might provide a function that’s just a no-op.

So that’s the standard binding protocol, and that’s what IronSmalltalk, IronPython and IronRuby do. And it’s what most languages do when they use the DLR’s DynamicMetaObject Protocol.

Of course, the point of that canonical (“officially recommended”) protocol is to ensure that objects have the right of first refusal in determining the concrete semantics of the abstract operations applied to them. And that’s a commendable and worthy goal of any programming language and/or execution environment.

But it’s not what Essence# does.

The Essence# Dynamic Binding Protocol

1. The first step of the Essence# dynamic binding protocol is identical to that of the canonical protocol. It pretty much has to be: The code that implements it is part of the DLR itself. Although it would be possible to circumvent it but still use the DLR’s DynamicMetaObject Protocol for dynamic binding, it would be a lot more work, and would be much more likely to break if the DLR’s API is ever changed.

2. If the receiver of the message is a native Essence# object, then the object’s class will be asked for the method whose selector matches the message that was sent. If the class has such a method (either in its own local method dictionary, in the method dictionary of the trait or composite trait it’s using, or if its superclass can provide a matching method,) then that method will be used as the binding’s invocable function, and the receiver’s CLR type and Essence# class will be used as the binding restriction (actually, the version ID of the class will be used, because the method might be removed from the class at some time in the future.) If no matching method can be provided by the receiver’s class, then the binding subsystem will ask the class for its #doesNotUnderstand: method. If it can provide such a method, then that will be used. Otherwise, it will provide a function for the binding that directly raises the MessageNotUnderstood exception (so defining the #doesNotUnderstand: message is entirely optional.)

3. If the receiver of the message is not a native Essence# object (i.e., it’s an instance of some non-Essence# CLR type,) then the Essence# Object Space for that execution contextwill be asked to find or dynamically construct the Essence# class that represents instances of that CLR type. That operation cannot fail–a new Essence# class will always be defined, if it does not already exist. And there can be only one such class for each CLR type in any particular Essence# Object Space. The object’s class will then be asked for a method that matches the message selector, which happens in the same way as it would for a native Essence# object. The only difference is what happens if the class cannot provide a matching method:

4. In cases where the Essence# class for a foreign object does not have a method that matches the message selector, the DynamicMetaObject of the receiver will then be asked to provide a binding. If it is able to do so, then that binding will be used.

5. Otherwise, if the receiver’s DynamicMetaObject fails to provide a binding, the Essence# dynamic binding subystem takes over the responsibility for doing so. And that’s where much of the magic happens which enables Essence# to interoperate so smoothly and effectively with languages that aren’t primarily based on the DLR (i.e, C#, F#, VisualBasic, etc.) That’s also where you’ll find the bulk of the code that implements the Essence# run time system.

So that’s what Essence# does to implement dynamic binding. The remaining question is why it does it that way, even though it’s not what the authors of the DLR strongly recommend.

The reason is both quite simple, and quite profound: The difference between the standard naming conventions of different programming languages in general, and the very stark differences between the naming conventions of Smalltalk-like languages and every other programming language in existence.

You see, the DLR’s protocol is based on the fallacy that the names of operations are the same in different languages, and based on the false assumption that what one language does by using a syntactical construct other languages also do by using some syntactical construct. Neither of those assumptions hold universally even between languages that use the same operation naming syntax, such as identifier + [“(” + {argument} + “)”] + “;”. But of course, Smalltalk-based languages don’t even share that meta-syntax with other languages, let alone the names themselves. And Smalltalk-based languages do almost everything by sending messages, and almost nothing by using special syntax, such as appending “()” to indicate “invoke a function” or prepending “(typeName)” to convert a value from one type to another.

So that’s why the first step Essence# takes in finding a binding when a Smalltalk message is sent to a foreign object is to ask the object’s Essence# class to interpret the message: It’s an Essence# (Smalltalk) message, not a message whose name or syntactical form is highly likely to mean anything to the objects of any other language.

What could or should the objects of any other language do when asked to find a binding for messages such as #displayOn:at:, #~~, or even just #value? Who could reasonably expect a CLR delegate with no arguments to know that it should respond to that message by invoking itself?

Worse, message (method) names such as #~~ and #displayOn:at: aren’t even legal in other languages. Even if they were, they aren’t likely to have the same semantics, because different languages usually have different standard libraries which use different names for the same thing.

Yes, Microsoft’s standard languages for .Net all use the same standard library-but that’s a special case. The DLR was intended to be used by dynamic languages with pre-existing standard libraries that are quite different than .Net’s BCL, and was not intended to be used by languages designed and implemented by Microsoft as native residents of the .Net framework. It therefore was a mistake to design the CLR’s dynamic binding protocol based on that false assumption. And that’s true without even considering the difference between Smalltalk’s keyword message names and the naming conventions used by all other languages.

The bottom line is that the Essence# dynamic binding protocol handles both the difference between Smalltalk’s message name syntax and also the difference between Smalltalk’s traditional and canonical message name semantics and that of the .Net BCL, and does so in a way that minimizes the risk that a message name will be misinterpreted just because it accidentally matches the name of a foreign function.

Essence# also provides a solution to the inverse problem of Essence# (Smalltalk) objects going on excursions to the homelands of foreign programming languages, and having foreign operations applied to them. And I admittedly haven’t documented that yet. Nevertheless, this post is already too long, so I’ll leave that topic to another time.